A heating element in which a plurality of interconnected heating conductors is laid between two electrodes is known from DE 101 12 405. In case one of the heat conductors breaks, the networking of the heat conductors among one another leads to a diversion of current around the breakage point. Despite its breakage, this heating element 1 will continue to uniformly heat the surface to be heated. Precisely because of the networking of the heating conductors among one another, undesired concentrations of current can arise when an electrode breaks. Thereby overheating may occur in this region.
Heating elements with one or more contact conductors that are electroconductively connected to one another by several heating conductors are known. These heating conductors and/or contact conductors can consist, for instance, of copper or another suitable conductor material with sufficient electrical conductivity and can be shielded and/or reinforced, if desired, by an outer insulation. Conductors made of copper, however, can be stressed mechanically only to a limited extent, so that malfunctions due to material fatigue and/or breakage may occur. This is due primarily to the copper material's insufficient fatigue strength under reversed bending. Breakage of contact or heating conductors can occur in such heating elements. In this case, an interruption of the supply of electrical power occurs at the breakage point. The heating element 1 is then no longer functional, at least in the areas through which current no longer flows.
Contacting a plurality of heating conductors to a plurality of contact conductors, in order thereby to create redundancy in case of the failure of a conductor, is known from DE 41 01 290. There are application cases, however, in which the heating elements described there are still not sufficiently robust.
A heating element is known from DE 10 2004 037 410.4, in which the heating conductors are grouped into several mutually independent bands in order to reduce the likelihood of hot spots in case of electrode breakages. This problem does not arise in the present case, however, since the heating conductors prevent overheating by the very fact that they melt.
Coating copper conductors with silver to protect them against corrosion is known. If the applied silver is not free of pores, however, the copper can still be attacked. Moreover, silver diffuses into the copper over time. Thereby an Ag—Cu alloy is formed that is very brittle. Breakages of this boundary layer form the beginnings of cracks, which may also put the conductor at risk.
So-called clad wires, in which thread-like electrical conductors having a cladding of copper are provided with a steel core, can be used to remedy this problem. A clad wire consisting of a cladding made of platinum and a core of a noble metal material is known from DE 38 32 342 C1. The core can be adapted for criteria such as flexibility, tear and tensile strength, and reversal bending strength, while the cladding can be optimized for the desired electrical properties.
A clad wire with a core of stainless steel wire and a cladding of copper is known from DE 196 38 372 A1. Finally, a clad wire in which the cladding is made of steel and the core of copper, or optionally, the reverse, is described in DE 102 06 336 A1.
An important disadvantage of this material combination is the relatively high cost and only limited corrosion resistance of the clad wires. The copper sheath does conduct the electric current sufficiently well for most applications, but for many purposes it is not sufficiently resistant to corrosion.
A heating conductor in which a plurality of carbon fibers is sheathed by a shrink-fit tube is known from JP 2001-217058.
A heating blanket in which a heating conductor is furnished with a layer that melts at impermissibly high temperatures and interrupts the heating current is known from DE 698 06 636 T2. The heating conductors described there are relatively thick, however, and therefore not suited to vehicle seats since they interfere with comfortable sitting.
An electrical heating element, in which plastic surfaces with a metallic coating are used as heating conductors and/or contact conductors, is known from WO 2005/089019 A2. In order to prevent fires in the surroundings, such heating elements self-destruct when a defined temperature is exceeded. Unfortunately, undesired initiation of this self-destruction effect can occur due to material fatigue, so that such heating elements sometimes have an undesirably short service life.